1. Field of the Invention
The present invention relates generally to electrical power conversion, and particularly to a fault ride-through and power smoothing system that enhances the capacity of a photovoltaic power generation system equipped with supercapacitor energy storage to ride through a grid fault to smoothly maintain power at the grid or load.
2. Description of the Related Art
The increased level of photovoltaic array penetration into the power system has resulted in the revision of Grid Codes for photovoltaic generators in many countries. Grid codes issued during the last few years invariably demand that wind farms and PV systems must withstand voltage dips to a certain percentage of the nominal voltage (down to 0% in some cases) and for a specified duration. Such requirements are known as Fault Ride Through (FRT) or Low Voltage Ride Through (LVRT), and are described by a voltage vs. time characteristic curve.
Different countries have their own grid codes to integrate PV systems, such as IEC 61727, which specifies PV systems characteristics of the utility interface. VDE-AR-N 4105 specifies for PV and other power generation systems the requirements for low voltage (LV) distribution network integration, and the German Association of Energy and Water Industries BDEW specifies the requirements for medium voltage (MV) distribution networks.
One concern of using PV as a power generation unit is that its output power is unpredictable as a result of varying irradiation and temperature. To overcome this fluctuation, different energy storage devices are integrated to the PV system. Among these, battery and supercapacitor energy storage systems (SCESS) are used in the literature because of their high energy and power densities, respectively.
Some work has been done to integrate SCESS with STATCOM for low voltage and fault ride-through, as well as smoothing power fluctuation of wind energy systems. In many existing smoothing systems, DC link voltage is assumed constant. In reality, due to changing temperatures and irradiance in photovoltaic systems, DC link voltage is rarely constant.
Thus, a fault ride-through and power smoothing system solving the aforementioned problems is desired.